As methods of drilling copper-clad laminates used for printed wiring board materials, there is generally adopted a method in which copper-clad laminates are stacked, an aluminum foil or the like is disposed on the top of the resultant stack and a hole is drilled.
In recent years, in accordance with a demand for improvement in reliability or an advance of high-densification concerning printed wiring board materials, high-quality drilling is required. For example, an improvement in hole position accuracy or a decrease in inner wall roughness is required. For coping with the above requirements, a hole-making method in which a sheet of polyethylene glycol is used (JP-A-4-92488), a lubricant sheet for making holes obtained by forming a water-soluble resin layer on a metal foil (JP-A-6-344297) and an entry sheet for making holes obtained by using an aluminum foil having a thermosetting resin thin film formed and forming a water-soluble resin layer made of a solventless water-soluble resin composition on the thermosetting resin thin film of the aluminum foil (for example, JP-A-2003-136485) have been proposed and practically adopted.
As methods for forming resin layers in these entry sheets for making holes, proposed are a method in which a resin composition is mixed with kneading means such as a roll or a kneader and a resin layer is formed on a base material by a roll process or a curtain coating process and a method in which a resin composition is molded with a press, a roll or a T-die extruder into a resin sheet having a desired thickness in advance, the resin sheet is laminated on a base material, and then the resin sheet and the base material are adhered to each other under heat with a press or a roll or bonded to each other, if necessary, with an adhesive or the like.
However, these methods have problems about the complicacy of steps, an alteration due to heat during melting, and a limitation on the range of a thickness due to difficulty in forming a thin resin layer. In regard to a method in which a low-boiling-point organic solvent solution of a water-soluble resin composition is applied and dried, there are problems about the limitation of the water-soluble resin which can be dissolved in organic solvent or the necessity of an exhaust gas treatment.
Further, a method in which an aqueous solution of a water-soluble resin composition is applied to a base material and dried to form a resin layer is also known. However, this method has a problem about a deterioration in hole position accuracy because of remaining bubbles in the resin layer or the occurrence of continuous ridges on the surface of the resin layer. In particular, when holes having a small diameter of 0.2 mm or less are drilled, the remaining bubbles or the ridges on the resin layer surface deteriorates hole position accuracy since a burden imposed on the drill in resin portions and a burden imposed in bubble portions are not uniform during drilling. Therefore, a further improvement is required.
The remaining bubbles in the resin layer or the occurrence of the continuous ridges on the resin layer surface is caused as follows. The melting points of water-soluble resins used for entry sheets are lower than the boiling point of water in many cases, so that drying due to vaporization of water and self-melting due to heat continuously occur in the step of drying of an aqueous solution of such a water-soluble resin applied to a base material and a resin layer is finally formed by cooling-solidification. Therefore, problems such as removal of bubbles in the resin layer or the formation of a flat smooth resin layer surface occur, so that it is difficult to perform stable production. With regard to a water-soluble resin having a melting point higher than the boiling point of water, on the other hand, a drill-cooling effect due to melting is low during drilling, so that the effect of such a resin is low as a resin for entry sheets.